1. Field of the Invention
The invention relates to a feed-through coil arrangement for use in a test apparatus for testing long products in a feed-through method by means of eddy current, to a test method for testing long products by means of eddy current as well as to a test apparatus having such a feed-through coil arrangement.
2. Prior Art
Long products are elongated metallic objects, such as wires, bars, rods or tubes or the like, for example. Such long products can serve as starting materials for high quality end products and are frequently subject to extremely high quality requirements. Testing for material flaws or faults, for example for cracks near to the surface, cavities, scabs or other material inhomogeneities (in the following also referred to as flaws or defects), forms an important part of the quality control of these products. In this context, a most complete testing of the material surface with a high resolution is generally aimed at, which testing is to be capable of being carried out where possible at the manufacturing site in synchronism and at the speed of the manufacturing process. Nowadays such tests are often carried out using electromagnetic methods, in particular eddy current technology, in a feed-through method. During a test using a feed-through method, an object to be tested (test specimen, test object) is moved at a predefinable, where required relatively high, feed-through speed through a test section of a test apparatus equipped with the corresponding sensor system, and the object is tested in the process.
During the non-destructive testing of materials using the eddy current method, an exciter coil operated with alternating current is used to induce an electrical alternating current (eddy current) with a suitable orientation, magnitude and frequency in the material to be tested, and the irregularities which are produced in the eddy current are detected and evaluated using sensors, for example a coil arrangement.
In eddy current testing, use is made of the effect according to which most impurities or defects in an electrically conductive material have an electrical conductivity and/or permeability different from that of the test material per se. The measurement signal to be evaluated is determined in particular from the conductivity and permeability of the material of the test specimen and from the distance between the eddy current sensor and the material surface, wherein the absolute strength of the fault signal and also the ratio between the useful signal and interference signals (signal-to-noise ratio, S/N ratio) decrease as the distance of the sensor from the material surface increases.
In one class of test apparatuses for the feed-through method, a feed-through coil arrangement comprising the test object is used through which arrangement the object to be tested (the long product) is fed through. A feed-through coil arrangement comprises an exciter coil arrangement with an exciter coil, which surrounds a passage opening for feeding through an elongated object along a feed-through direction. The exciter coil arrangement comprises a connection device for connecting the exciter coil to an alternating current voltage source. Furthermore, a receiver coil arrangement arranged around the passage opening is provided, which comprises a connection device for connecting the receiver coil arrangement to an evaluation device of the test apparatus. The exciter coil arrangement and the receiver coil arrangement are connected to the electric or electronic components of the test apparatus via the connection devices. Such comprehensive feed-through coil arrangements may generally be produced relatively cost-efficiently and are reliable and efficient in use even under tough environmental conditions due to their robustness.
In the patent specification DE 101 35 660 C1, such a feed-through coil arrangement is described, that would allow economic and reliable detection of path-type faults, to ensure a clear fault resolution and a good reproducibility of the test result. The feed-through coil arrangement comprises a measuring element surrounding the object to be tested externally in the form of at least three segment measuring coils made of strip lines using a differential or multi-differential circuitry surrounding the object circularly at a medium radial distance. Said segment measuring coils overlap in the circumferential direction with their end sections adjacent to one another and are coupled to a multichannel evaluation electronic system. Furthermore, the segment measuring coils are surrounded by an absolute coil, which in turn is surrounded by an exciter coil assigned to all measuring coils. In configurative adaptation to the segment measuring coils, the absolute coil may also be segmented, wherein in that case, the segment absolute coils overlap with their end sections. As a result of the coil effective areas of the segment measuring coils overlapping in the circumferential direction, a real 100% coverage of the surface of the object to be tested at the same sensitivities is to be achieved.